Telephone circuit



Jan. 12, 1943.

R. LEE

TELEPHONE CIRCUIT Fig Z IWIILNESSES:

INVENTOR Reubeh Lee.

Patented Jan. 12, 1943 TELEPHONE CIRCUIT Reuben Lee, Catonsville, Md., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsyl- Vania Application July 12, 1941, Serial No. 402,158

Claims.

My invention relates to circuits useful in wire telephony, and in particular to arrangements for connecting two microphones so that a telephone transmission system may be controlled by either microphone or by both simultaneously at will.

It is sometimes required, for example, in radio telephone systems for police use, that it should be possible for two persons at different places to broadcast in rapid succession over the same transmitter. At other times it is desired that only one person should broadcast and that the microphone provided for the other person should be out out of circuit. It is, therefore, desirable to be able to provide a switch at each microphone by which it may be cut out or cut into circuit at the will of the speaker. One way to achieve this result would be to provide the control circuit of the transmitter or amplifier tube feeding power to the channel over which it is desired to transmit messages with two audio frequency transformers connected in series with each other, the primary circuits of the respective transformers being connected each to its own local line terminating at a microphone and switch. However, Where as is customary a direct current voltage supply is used for the microphone circuits, the

. audio frequency transformers would each be normally saturated to quite a high degree by the unidirectional current which would be flowing in them at all times when their appurtenant microphone switches were closed. The saturation thus resulting in the audio frequency transformer cores would make it necessary to build them unduly large and otherwise result in an expensive installation.

As one feature of my invention, I provide an arrangement in which the transformer secondaries are wound upon a single core bearing two primary windings, and the latter are so connected with the lines running to the microphones and with the direct-current energy source that when both microphone switches are closed, current flows from the source in such directions through the two primary windings that their magnetic effects on the transformer core neutralize each other. This makes it possible to greatly reduce the size and cost of the audio frequency transformer.

However, with an arrangement so simple as that shown in Fig. 1 as embodying the idea just outlined, the opening of one microphone switch will leave direct current flowing through one of the primary windings and this will saturate the transformer core thereby greatly reducing its effective permeability. In accordance with a further feature of my invention, I provide as in Fig. 2 a resistance network between th transformer primary windings and the microphones which insures that at no time will there be a substantial amount of unbalanced direct current flowing in the transformer primaries to cause saturation in the transformer core.

One object of my invention is, accordingly, to provide a circuit for connecting two microphones each having a local switch.

Another object of my invention is to provide an arrangement capable of effecting the object described in the preceding paragraph with a minimum of size and cost for the audio frequency transformer employed.

Still another object of my invention is to provide a network for achieving the object described in the two immediately preceding paragraphs with a minimum of expense in first cost and in subsequent operation.

With the foregoing objects in mind, my invention may be understood from the following specification taken in connection with the drawing, of which Figure 1 is a circuit useful in explaining my invention, and

Fig. 2 shows the circuits embodied in the preferred' form of my invention to connect the microphones with an amplifier which supplies the transmission channel.

Referring in detail to Fig. l, a pair of electronic amplifiers l and 2 is connected to a speech transmission line 3 in conventional push-pull arrangement. The control electrodes of the amplifier tubes are respectively connected to the end terminals of the secondary winding 4 of a transformer 5. The mid terminal of the primary winding 6 of the transformer 5 is connected to one pole of a direct-current source 1, the other terminal of this source being connected by one line conductor of two local telephone transmission circuits 8, 9 extending to the two different points from which it is desired to broadcast over the transmission line 3, to one pole each of a pair of microphone switches H, 52. The other terminals of the switches I I and I2 are connected through microphones is, it to the other line conductors of the local transmission circuits just mentioned. The opposite ends of the two line conductors just mentioned are connected to the free terminals of transformer primary 6.

The arrangement of Fig. 2 differs from Fig. 1

only in that resistors I 5, it which are preferably,

although not necessarily, made equal in value are connected between the outside conductors of 7 lines 3, 9 and the outside primary terminals of I transformer 5; and that a pair of resistors I1, l8 are bridged across the ends of lines 8, 9 adjacent transformer 5. a

Th resistors ii and it are preferably made equal to each other and are each preferably made approximately equal to the resistance of their respective microphones i3, I 4. The resistors [5, it are likewise preferably made equal to each other and also preferably are each large com- 1 and 2.

pared with the resistance of the above-mentioned microphones. The reactance of the primary winding of transformer at the lowest audio frequency to be transmitted is made approximately equal in ohms to the sum of the ohmic resistances of resistors l5 and IS.

A moments consideration will show that when both the microphone switches II and I2 are closed, substantially equal currents, except for the effects of speech modulation imposed by one or the other of the microphones l3, M, will flow in opposite directions through the primary windings of the transformer 5, and that since .these windings are preferably equal to each other in magnetic effect, the magnetomotive force tending to saturate the core of transformer 5 is substantially zero. The transformer 5 will, accordingly, be in a magnetic condition best adapted to transfer speech energy without distortion from the microphones to the amplifier tubes I If now it is desired not to' use one of the microphones, let us say I4, the switch I2 is opened, disconnecting itfrom the circuit. However, by reason of the fact that the resistance of the resistor I8 is small compared with that of the resistor IS, the reduction in current flow through the lower half of primary winding 6 will be relatively small, and hence the unbalancing effect in reducing magnetic saturation in the core of transformer 5 of the difference in the current in the two halves of primary winding 6 will be relatively small. The effect on the effective voltage of one microphone (say) I4 which results from closing or opening the switch (e. g., H) in circuit with the other is minimized by the Fig. 2 connections.

To give a specific example of the values of resistance suitable-for use in carryingout my invention, the resistors I7 and i8 may each be 50 ohms when the internal resistance of each microphone is approximately 50 ohms. The resistance of .the resistors i5 and I6 may each be 150 ohms.

The arrangement of Fig. 2 practically eliminates hum in the transmission channel arising from momentary fluctuations, such as commutator ripple, in the voltage of source 1. Hence expensive filtering of said source is unnecessary.

I claim as my invention:

1. In combination with a telephone transmission channel and two microphones located at separate points, a switch in series with each microphone, a pair of conductors connecting each microphone and its switch with the respective members of a pair of serially-connected re- K sistors, an audio frequency transformer having a pair of primary windings, a source of direct current connected between the adjacent terminals of said primary windings and the common terminal of said serially-connectedresistors, a pair of resistors respectively connecting the nonadja'cent terminals of said primary windings with the non-adjacent terminals of said seriallyconnectedresistors, and a secondary winding on said transformer connected to a speech transmission channel.

2. Incombination with a telephone transmission channel and two microphones located at separate points, a switch in series with each microphone, a pair of conductors connecting each microphone and its switch with the respective members of a pair of serially-connected resistors, an audio frequency transformer having a pair of primary windings, a source of direct current connected between the adjacent terminals of said primary windings and the common terminal of said serially-connected resistors, a pair of resistors large compared with said. microphones respectively connecting the non-adjacent terminals of said primary windings with the non-adjacent terminals of said serially-connected resistors, and a secondary winding on said transformer connected to a speech transmission channel.

3. In combination with a telephone transmission channel and two microphones located at separate points, a switch in series with each microphone, a pair of conductors connecting each microphone and its switch with the respective members of a pair of serially-connected resistors, said resistors being approximately equal in ohmic value to their appurtenantmicrophones, an audio frequency-transformer havinga pair of primary windings, a source of direct current connected between the adjacent terminals of said primary windings'and the common terminal of said serially-connected resistors, a pair of re sistors respectively connecting the non-adjacent terminals of said primary windings with the non-adjacent terminals of said serially-connected resistors, and a secondary winding on said transformer conn cted to a speech transmission channel.

4. In combination with a telephone transmission channel and two microphones locatedat separate points, a switch in series with each microphone, a pair of conductors connecting each microphone and its switch with the respective members of a pair of serially-connected resistors, said resistors being approximately equal in ohmic value to their appurtenant microphones, an audio frequency transformer having a pair of primary windings, a source ofdirect current connected between the adjacent terminals of said primary windings and the common terminal of said serially-connected resistors, a pair of resistors respectively connecting the nonadjacent terminals of said primary windings with thenon-adjacent terminals of said serially-connected resistors, the last-mentioned resistors having an ohmic value large compared with said microphones, and a secondary winding on said transformer connected to a speech transmission channel.

5. In combination with a telephone transmission channel and two microphones located at separate points, a switch in series with each microphone, a pair 'of conductors connecting each microphone and its switch with the respective members of a pair of serially-connected resistors, said resistors being approximately equal in ohmic value to their appurtenant microphones, an audio frequency transformer having a pair of primary windings, a source of direct current connected between the adjacent terminals of said primary windings and the common terminal of said serially-connected resistors, a pair of resistors respectively connecting the nonadjacent terminals of said primary windings with the non-adjacent terminals of said seriallyconnected resistors, the last-mentioned resistors having an ohmic value large compared with said microphones, the leakage reactance of said primary windings at the lowest audio frequency to be transmitted being approximately equal in ohms to the sum of the resistanceof the lastmentioned resistors, and a secondary winding on said transformer connected to a'speech transmission channel.

REUBEN LEE. 

